Petrography, mineralogy, and origin of the syenite pegmatitic segregation veins from La Peña alkaline complex, Mendoza, Argentina

Abstract

The syenite pegmatitic segregation veins (SPSV) of the Early Miocene La Pẽna alkaline complex (LPC), Mendoza, Argentina, are emplaced in a malignite body. They occur as veins, parallel layers, and small elliptical bodies 1 to 6 cm wide and 0.2 to 1 m in length. The veins with a pegmatitic to micropegmatitic texture have a thin dark border composed of andradite, potassic-hastingsite, and dark micas and a thicker internal zone of poikilitic Ba-Sr-poor K-feldspar (Or90.3Ab8.9An0.8), nepheline (Ne74.9Ks19.4Qz5.7 to Ne78.7Ks20.0Qz1.3), potassic-hastingsite, dark mica, andradite, scarce clinopyroxene (diopside- hedenbergite), and locally zoned microphenocrysts of Ba-Sr-rich K-feldspar (Or75.6Ab21.7An2.7). Accessory fluorapatite, titanite, interstitial sodalite, and magnetite and secondary abundant Na- And K-rich zeolites, calcite, chlorite-group minerals, and possibly cancrinite have been identified. The textural and mineralogical characteristics of the SPSV show genetic links with the host malignite that belongs to the potassic alkaline series. The SPSV were formed from a residual syenitic melt (malignite) extracted from a parental tephritic magma. In the residual malignite, the concentration of primary crystals (apatite, clinopyroxene, Ba-Sr-rich K-feldspar, and nepheline) formed a crystal network (mush) with the open spaces occupied by small amounts of residual melt enriched in Al, Na, K, and volatiles and depleted in Ba and Sr. Deformation (by shear movements plus magmatic pressures) produced microfractures and fractures which were occupied by residual interstitial melt. In situ crystallization within these fractures resulted in the formation of the SPSV.